Field
The present disclosure relates generally to the field of thin-film sol-gel coatings and in particular to coating on substrates such as glass or solar panels.
Description of Related Art
A significant reliability concern for photovoltaic (PV) solar modules is potential induced degradation (PID). PID is caused by interactions between the environment (heat, humidity) and elements within the PV solar module that lead to a degradation in the output power of the PV solar module over time. It is generally believed that PID occurs when the module's voltage potential and leakage current drive ion mobility within the module between the semiconductor material and other elements of the module (e.g. glass, mount and frame) causing the module's power output capacity to degrade. The ion mobility accelerates with humidity, temperature and voltage potential. Tests have revealed the relationship of mobility to temperature and humidity, Pingel, S., et al. “Potential Induced Degradation of solar cells and panels.” Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE. Some studies have identified the soda lime glass used in the PV solar panel as the source of positive sodium ions and other metallic ions that may be the prime cause of PID; Schutze, M., et al. “Laboratory study of potential induced degradation of silicon photovoltaic modules.” Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE; Hacke, Peter, et al. “System voltage potential-induced degradation mechanisms in PV modules and methods for test.” Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE.
In general, susceptibility to PID can be reduced by minimizing leakage currents induced by the high potential on the module. Two approaches have been useful in reducing susceptibility to PID. First, the encapsulating materials used to bind the glass to the solar cells and back-sheets have been shown to have a significant effect on PID. Encapsulating materials such as ethylene-vinyl acetate (EVA) with low resistance to moisture intrusion or with low electrical resistance are particularly problematic. Therefore, PV module manufacturers have tested advanced encapsulating material such as Enlight™ polyolefin from Dow Chemical, which are formulated to eliminate PID. However, these materials tend to be significantly more expensive than the competing materials such as EVA, which are more PID prone. Second, studies have shown that the anti-reflective, passivating coating applied to the PV cell itself can have a significant effect. This has led to approaches to engineer protective cell coatings that can reduce PID effects such as silicon nitride cell coatings without porosity using methods that sometimes include increasing density and/or thickness. One complication of silicon nitride modification is that it also sub-optimizes refractive index or complicates process control and cost. Increasing the silicon nitride thickness increases cost and may also reduce overall module conversion efficiency.
U.S. patent application Ser. No. 13/690,954 teaches that a hydrophobic coating covering at least a portion of the frame and at least a portion of the glass sheet may help prevent PID. Also Tatapudi, S. R. V. (2012) “Potential Induced Degradation (PID) of Pre-Stressed Photovoltaic Modules: Effect of Glass Surface Conductivity Disruption” (master's thesis). Arizona State University, Tempe Ariz., teaches that a hydrophobic but high transmittance surface coating (such as Teflon) could disrupt the surface conductivity of the glass by modifying the frame/edges with water repellent properties.
Several references discuss sodium barriers that prevent or retard the migration of sodium out of soda-lime glass to solve several problems. U.S. Pat. No. 8,217,261 teaches a sputtered or sol-gel deposited SiO2 barrier layer between soda-lime glass and the molybdenum electrode layer in a CIGS PV solar module. The barrier layer prevents the migration of too much sodium into the CIGS absorber layer caused by thermal processing during manufacture. Nocuń, Marek, et al. “Sodium diffusion barrier coatings prepared by sol-gel method”, Optica Applicata, Vol. XXXVIII, No. 1, 2008, describes various spin-coated sol-gel barrier layers designed to protect the tin or indium conductive transparent thin-film layers that are used in some electronics devices from migration of sodium from the glass substrate. Yan, Yanfa, et al. “SiO2 as barrier layer for Na out-diffusion from soda-lime glass” Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE, found that a sputtered SiO2 layer could be an effective sodium barrier for F-doped SnO2 coated soda-lime glass as used by commercial CdTe thin-film PV solar module manufacturers to prevent sodium out-diffusion caused by thermal processing at 550° C. to 650° C. Rose, Doug, et al. “Mass production of PV modules with 18% total-area efficiency and high energy per peak watt,” Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, stated that a 4-layer sputtered anti-reflective coating called PV-Lite manufactured by Saint Gobain Glass prevented leaching of alkalis from the glass that could cause degradation of the glass surface. Hacke, Peter, et al. “Test-to-Failure of Crystalline Silicon Modules,” Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE found that modules with an AR coating on the glass exhibited better power retention through PID testing with negative bias than those without such a coating and suggested that this was consistent with the use of oxide barrier layers in thin-film modules that are used to minimize sodium migration that causes degradation. Hacke, Peter, et al. “Characterization of Multicrystalline Silicon Modules with System Bias Voltage Applied in Damp Heat,” 25th European Photovoltaic Solar Energy Conference (EUPVSEC) 2010, found that soda-lime glass with an added SiO2 barrier, as used by thin-film PV industry to mitigate sodium migration, performed in an intermediate manner (compared to glass without such a barrier), but still showed significant degradation during negatively biased PID testing. The paper concluded that higher electrical resistance packaging will help reduce electrolytic and ion migration-related degradation, which can be controlled by choices including the glass and its coatings, the encapsulant and the frame.
PID remains a significant problem for PV solar module reliability. It would therefore be advantageous if there were low cost materials and methods to eliminate PID without adversely impacting solar conversion efficiency.